Lead alloy

ABSTRACT

A LEAD ALLOY ANODE WITH IMPROVED RESISTANCE TO CORROSION IS PROVIDED FOR SULFATE ELECTROLYSIS WHICH ANODE CONSISTS ESSENTIALLY BY WEIGHT OF NOT MORE THAN 5% THALLIUM, NOT MORE THAN 1.8% TIN, UP TO 0.2% COBALT AND THE REMAINDER LEAD AND INCIDENTAL IMPURITIES. FURTHER IMPROVEMENT RESULT OF THE CAST ALLOY ANODE IS &#34;HOT WORKED&#34; BY ROLLING THE ANODE TO LESS THAN ONE-QUARTER ITS ORGINAL CAST THICKNESS WHILE MAINTAINING THE TEMPERATURE BETWEEN 200*-300*C.

United States Patent 3,664,832 LEAD ALLOY Neil E. Paton, Cambridge, Mass., assignor to Ionics, Incorporated, Watertown, Mass. No Drawing. Filed July 25, 1969, Ser. No. 845,073 Int. Cl. C22c 11/00 US. Cl. 75-166 D 2 Claims ABSTRACT OF THE DISCLOSURE A lead alloy anode with improved resistance to corrosion is provided for sulfate electrolysis which anode consists essentially by weight of not more than 5% thallium, not more than 1.8% tin, up to 0.2% cobalt and the remainder lead and incidental impurities. Further improvement result if the cast alloy anode is hot worked by rolling the anode to less than one-quarter its original cast thickness while maintaining the temperature between 200-300 C.

The present invention relates to novel lead alloys having improved resistance to the corrosive action of acids particularly when used as an anode in electrolytic sys terns and further concerns the method of making and using the same. More particularly the invention relates to low cost, stable lead alloy anodes useful in the electrolysis especially of sulfate solutions and also useful in the electrodeposition and electrowinning of metals such as zinc.

In electrochemical processes such as electrolysis, electrodialysis, cathodic protection and the like, lead or lead alloy materials have been employed as the anode material. The preferred lead alloys were cast into anodes usually containing silver. When employed in the electrodialysis of alkali metal sulfate solutions to produce sulfuric acid and caustic, it was found that these anodes corroded substantially and would undergo an appreciable decrease in weight as the electrolysis proceeded. During electrolytic operation the decomposition products formed on the anode would flake off and eventually prevent further use of the electrode and replacement of the same.

It is therefore the object of this invention to provide a stable, low cost lead alloy characterized by long life when used in contact with acid liquors.

It is also an object to provide an anode of such alloy which has greater resistance to electrolytic corrosion while in operation than the prior art electrodes.

Another object is to provide a superior alloy anode especially for use in the electrolysis of alkali sulfates, alkali acid sulfates, sulfuric acid and the like. Further objects will become apparent from the following description.

According to the present invention these stated objects are achieved by carrying out the electrolytic process with lead-thallium anodes which have been hot worked during manufacture and/or which preferably have had small quantities of other alloying elements added thereto. In the use of cast lead anodes containing thallium it has been found that the addition of small quantities of tin and cobalt has materially improved the stability of the leadthallium anode and further the use of a hot working ice process in fabricating the electrodes as contrasted to the simple casting process alone has a very pronounced effect in producing a more stable anode. In the use of such hot worked anodes in electrolytic sulfate solutions, there is at least a three fold decrease in weight loss when compared to the cast anodes as will hereinafter be disclosed. According to the invention there can be used a cast or hot worked anode which consists principally of lead with about 0.5 to 5% thallium by weight alloyed with small amounts of tin and cobalt, that is from about 0.3 to 1.8% tin and up to about 0.2% cobalt.

By way of example an alloy which is especially suitable for the purpose of the present invention consists of about 2% thallium, 0.45% tin, and 0.05% cobalt with the balance of the composition for the most part being lead, although trace amounts of incidental elements normally associated with lead are not precluded. An even more stable material is obtained if the above described alloy is manufactured into an anode by a hot working technique as will hereinafter be more fully described. For purposes of this disclosure hot working is defined as a method of cansing plastic deformation of the lead alloy composition by subjecting the alloy to an appropriate pressure and temperature to cause a change in its physical shape. This may be accomplished by subjecting the cast alloy sheets or billets to rolling, hammering, extrusion and the like while retaining the temperature of the alloy material between about room temperature and its melting point. It has been determined that a hot working technique of rolling the cast lead alloy material down to about three-quarters or less of its original thickness while maintaining the temperature in the required range will produce anodes having unexpected improvements in stability. In general the greater the reduction in the dimensional thickness of a cast. sheet and/or the higher the temperature employed, then the more stable the anode produced. The preferred hot working method is to roll the cast alloy material down to on-equarter or less of its original thickness while maintaining a temperature between about 200 C. to 300 C.

The lead employed is of the usual commercial grade and the alloying materials of thallium, tin and cobalt of the purest commercial grades. A stock alloy of tin-cobalt in the desired ratio may first be prepared and then introduced into an alloy melt of lead-thallium. The composition of the stock alloy is checked by chemical analysis. The total mixture is carefully stirred and from the alloy thus prepared, anode sheets are cast in steel or carbon molds for investigating their stability during electrolysis. Upon solidification, a certain number of said cast sheets are hot worked, the preferred method involving rolling the sheets at elevated temperatures to a thickness which is only about one-quarter their original cast thickness. In the most preferred instances cast sheets of at least onehalf inch thickness are rolled down to about one-eight of an inch thickness at temperatures of between about 275 300" C. As will be hereinafter illustrated, the hot worked anodes withstand the corrosive action of electrolysis to a far greater extent than the prior art lead-thallium cast anodes and even show improvement over the cast quaternary alloy material (Pb-Tl-Sn-Co) of the present invention. For the purpose of testing the stability of the alloy both cast and hot worked sheets are cut into anodes;

(1" x 3") each constructed with a tab extension for an electrical connection. Several electrodes are prepared simultaneously, identical in composition and in casting or hot Working techniques.

The anodes thus prepared are weighed and placed in rectangular glass containers between nickel sheet cathodes and each test cell joined electrically in series. The electrolysis is performed in a test solution of sulfuric acid or sodium acid sulfate at a current density of 120 amps/ft. (1280 amps/meter of submerged anode working area and at temperatures of between 5565 C. Whenever possible at least two samples of each alloy, both cast and hot worked are examined simultaneously. The elec trolysis is carried on continuously with any loss of electrolyte being made up daily. After about each seven days of operation (20,000 ampere hours) the current is stopped, the anodes removed, washed, dried and carefully weighed. The samples are then replaced in freshly made test solution and operation continued for a total of at least 21 days (500 hours or 60,000 ampere-hours).

The invention is more fully appreciated by means of the following table, the results which clearly illustrate the superiority of the novel quaternary alloy and hot Working process of the present invention. These anodes are especially suited for use in an electrolytic apparatus and process for scrubbing S gas from flue gases as is disclosed in copending application Ser. No. 625,149 filed Mar. 22, 1967 in the name of W. A. McRae et al. In the table the weight loss of the samples are calculated in grams per square meter of anode area after 500 hours of total operation. From the data it can be readily seen that the quarternary alloy is more stable than the binary alloy of lead-thallium alone and the use of a hot Working technique in preference to casting produced an unexpected increase in stability in both the binary and quaternary alloy compositions. It will be further noticed that although the anodic weight loss is greater in the sulfuric acid test solution when compared to the sodium bisulfate solution, the improved stability of the present anodes is still evident in the sulfuric acid.

It should be mentioned that the various metallic ingredients entering into the improved alloy fuse at relatively low temperatures and the alloys are made without much difiiculty by first melting and mixing or by first mixing and then melting the various ingredient in the proper proportions.

TABLE Weight loss (grams per square meter) Norm-As will be appreciated from the foregoing data, the relative quantities and proportions of the alloying materials employed in the anodes may be considerably altered without departing from the full scope and spirit of the invention.

What is claimed is:

1. A quaternary alloy characterized by improved resistance to corrosion in aqueous electrolytes, said alloy consisting of between about 1 to 2 percent by Weight of thallium, between about 0.3 to 1.8 percent by weight of tin, between about 0.02 to 0.2 percent by weight of cobalt and the balance lead and incidental impurities.

2. The alloy of claim 1 characterized wherein the weight percent of thallium is about 2, the weight percent of tin is about 0.45, and the weight percent of cobalt is about 0.05.

References Cited UNITED STATES PATENTS 2,841,491 7/1958 Zahn -166 R 3,078,161 2/1963 Zahn 75-166 D 1,384,056 7/1921 Fink 75-166 R 2,419,722 4/1947 Lowe et al. 75-166 D FOREIGN PATENTS 185,074 1966 U.S.S.R. 75-166 R OTHER REFERENCES Kiryakov et al.; article in Inst. Khim. Nauk. Akad.

Nauk. Kaz. SSR, vol. 18, pp. 6-17, 1967, also pp. 1, 14-18, of Draft Transl.

HENRY W. TARRING II, Primary Examiner U.S. Cl. X.R. 75-166 R 

